There are several technologies which use rapid heating and cooling to generate unique micro structures and properties in various classes of materials. As an example, these include rapid solidification processing (RSP) and rapid thermal processing (RTP), interalia, of silicon wafers. RTP is of great importance because it appears that single-wafer and cluster-based tools will be the preferred manufacturing approach taken by the silicon device industry. The primary problem which exists, and which has existed for the past several years with the commercialization of these technologies, and the further spread of such technologies, is the non-availability of adequate (and low cost), area heating devices. Devices such as laser or lamps are point source devices and, therefore, cannot be properly or efficiently employed for the purposes required for RTP and RSP type applications. The molybdenum silicide-containing ceramic and metal ceramic products of the present invention, if used as heaters, enable one to effectively utilize RTP and RSP techniques. The term "molybdenum silicide", as used herein, refers to any silicide of molybdenum having the generic formula Mo.sub.x Si.sub.y. The unique and novel properties of the products of the present invention, specifically, the high emissivity of the products of the present invention and also the much greater resistivity, make them especially suitable for application in RSP and RTP type inventions, as will be described in greater detail below.
Preferably, the products of the present invention are manufactured using the technique described and claimed in copending application Ser. No. 08/027,710, filed Mar. 8, 1993, the corresponding PCT application of which was published as WO 94/20243 on Sep. 15, 1994 (herein after collectively referred to as "the '710 application"). This method comprises optimizing performance characteristics of a combustion synthesized ceramic or metal ceramic product, the product having thermal and mass gradients, a lowest melting phase having a melting point and said product having homogenous sections separated by non-homogenous sections, each said homogenous section being separated from the next homogenous section by an average repetitive distance, the product having been manufactured by blending a mixture comprising between about 5% and 95% by total weight of the mixture, of at least one reactive system, wherein said reactive system comprises at least two particulate combustible materials which will react exothermically with one another by combustion synthesis and are present in such proportion to one another that combustion of said mixture will occur when ignited, up to 95% by total weight of the mixture, of a filler material, and optionally a sufficient amount of a liquid phase in order to form a slurry, fashioning said mixture into a desired and uncombusted shape, and combusting said shape by ignition at a temperature between about 150.degree. C. and 1800.degree. C.; the method comprising the steps of: initially applying sufficient current to the product so as to heat the product to a minimum of 50% of the melting point in degrees Kelvin, of the lowest melting phase in the product, wherein the current applied is selected from the group consisting of a DC current, an AC current, a pulsed current and an induction current; and greatly reducing porosity of said product so as to make the repetitive distance between consecutive homogenous sections of said product to less than 0.002 m, by increasing said current applied so as to cause elimination of said thermal and mass gradients.
The referenced combustion synthesis, also known as micropyretic synthesis or self-propagating high-temperature synthesis (the term Micropyretic Synthesis is preferred for slurries), is a novel processing method for the production of intermetallics, engineering ceramics, metal-ceramics, and other materials. See U.S. Pat. No. 5,188,678 hereinafter referred to as the '678 patent. The technique employs exothermic reaction processing which circumvents difficulties associated with conventional methods of time and energy-intensive sinter processing. Complicated engineering gear shapes, such as shown in the '678 patent, have been successfully produced by this technique.
Two basic micropyretic synthesis modes are commonly employed, namely the wave propagation mode and the thermal explosion mode. In the wave propagation mode, the combustible compact is ignited at a point by a heat source. After ignition, the heat required to propagate the combustion wave is obtained from the heat released by the formation of the synthesized product. The unreacted portion in front of the combustion wave is heated by this exothermic heat, undergoes synthesis, the wave propagates, thus causing further reaction and synthesis. In the thermal explosion mode, the specimen is heated in a furnace. The furnace may be kept at the ignition temperature or the specimen may be heated in the furnace at a predetermined heating rate to the ignition temperature. The combustion reaction in this mode may occur more or less simultaneously at all points in the specimen. Although the combustion product phases obtained by both techniques are similar, there may be differences in the amount of residual porosity and the final dimensions of the synthesized part.
The advantages of micropyretic synthesis techniques include rapid net shape processing as disclosed in the '678 patent and clean products. When compared with conventional powder metallurgy operations, micropyretic synthesis not only offers shorter processing time, but also eliminates the need for high-temperature sintering. Volatile impurities or contaminants are expected to be expelled as the high temperature combustion wave propagates through the sample, and thus the synthesized products have high purity.
In most combustion synthesized products, porosity of the final product is often the most uncontrollable and deleterious drawback. It has been shown that porosity degrades mechanical properties in combustion synthesized parts. See, H. P. Li, S. Bhaduri and J. A. Sekhar, Metal Trans A 1992, vol. 23 p 251-261. Porosity increases with the proportion of the material which is combustible as this raises the combustion temperature. Porosity mainly develops from (a) the molar volume differences between reactants and products; (b) the porosity from the initial powder or slurry compact (see the '678 patent and United States Patent 5,279,737, hereinafter referred as the '737 patent); (c) gases adsorbed and absorbed in the initial reactants; and (d) the expansion, swelling and related pressure of gases in the initial compact.
The techniques for elimination of porosity from products of combustion synthesis have included the following: (1) the simultaneous synthesis and sintering of the product; (2) the application of external force or pressure during or soon after combustion. See e.g.: U.S. Pat. No. 4,909,842 and J. Puszynski, S. Majorowski and V. Hlavacek, Ceram Engn. Sci. Proc.,Vol. 11, p.1182,1990.; (3) the use of liquid phases in the combustion process to promote the formation of dense products. See e.g.: Z. A. Munir and U. Anselmi-Tamburini, Mater. Sci. Reports, vol. 3, p. 277, 1989; J. B Holt, S. D. Dunmead, Annual rev of Mater Sci. vol. 21 pg 305, 1991; Z. A. Munir, Amer Ceram Bull, Vol 67(2), pg. 342, 1988; H. C. Yi and J. J. Moore, J. Mater Sci., vol. 25, p 1159, 1992; J. Subramanyam and M. Vijaykumar, J. Mater. Sci. vol 27. pg. 6249, 1992; and U.S. Pat. No. 4,961,778 and U.S. Pat. No. 4,610,726 (not directed towards combustion synthesized samples but incorporating liquid phase for densification); and (4) lessening the gas evolution by outgassing the reactant compact prior to ignition.
All the techniques mentioned above have serious drawbacks. Combustion processes are rapid by nature and the time for simultaneous sintering is always too short to be of value.
External force or pressure is difficult to apply in most situations involving combustion, a point made in detail in the '678 patent. Additionally, such application of pressure (normally as high as 0.3 GPa) limits this method to use with simple shapes like cylinders and to situations where die damage is not a problem. Die damage invariably occurs when pressure is applied at the high temperatures usually associated with combustion synthesis. In fact to overcome the problems and high cost associated with die damage, several variations of the pressure techniques have been developed. These include (a) hot pressing immediately after the combustion wave; (b) hot rolling behind the combustion wave; (c) high pressure during combustion; and (d) shock wave consolidation with explosives or by dynamic compaction. See Z. A. Munir and U. Anselmi-Tamburini, Mater. Sci. Reports, vol. 3, p 277, 1989; J. B Holt, S. D. Dunmead, Annual Rev. of Mater. Sci. vol. 21 p 305, 1991; and Z. A. Munir, Amer Ceram Bull, Vol 67(2), p 342, 1988; H. C. Yi and J.J. Moore, J. Mater Sci., vol. 25, p 1159, 1992; J. Subramanyam and M. Vijaykumar, J. Mater. Sci. vol 27. p 6249, 1992; S. D. Dunmead, Z. A. Munir, J. B. Holt and D. D. Kingman, Combustion and Plasma Synthesis of High Temperature Materials, Z. A. Munir and J. B. Holt eds., VCH Publishers, New York, p 229, 1990; J. B. Holt, Mater Research Bull., Vol 12(7), p 60, 1990; and L. J. Kecskes, T. Kohke and A. Niler, J. Amer Ceram. Soc., vol. 73, p 1274, 1990; L. J. Kecskes, R. F. Benk and P. H. Netherwood Jr., J. Amer Ceram Soc., Vol. 73, p 383, 1990. Notably, however, none of the techniques developed to avoid die damage have eliminated this problem.
If a liquid phase is involved then it is hoped that this liquid phase will wet the products and will fill the porosity which is formed during the combustion, thereby leading to the formation of denser products. Unfortunately this technique has several drawbacks including the fact that most often the liquid does not wet the products. The volume of the liquid may not be enough to fill the pores and the residence time of the hot liquid again may be too short to fill the pores in time. To improve such drawbacks simultaneous liquid formation and pressure application has been tried by centrifuging the part, but with limited success. See e.g. P. Odowara, J. Amer Ceram. Soc. Vol. 73(3), p 629, 1990. In any case, 130 G's (a `G` is the unit for acceleration due to gravity.about.9.81MN/m.sup.2), had to be applied for making the liquid enter the pores. Such a high applied acceleration limits the size of the part that may be densified and additionally limits the kind of material which may be densified. These problems are distinct from the complexity that would be required for an experimental system which would be capable of applying such accelerations at the high temperatures at which combustion synthesis typically occurs.
The simplest method to eliminate the porosity would really have been a high temperature sintering of the combustion synthesized part in a suitable furnace. If at all possible, the application of this technique is limited in situations where densification has to occur at temperatures well above 1500.degree. C., because of the limited availability and the small sizes of furnaces at such high temperatures. Although the furnace sintering method is simple, there are other drawbacks to such an operation, aside from the high costs normally associated with furnace sintering. The real disadvantage of such a technique lies in the fact that the agglomerated porosity (such as always obtained in combustion synthesis) cannot be eliminated in this manner to obtain full density. See e.g. B. Kellet and F. Lange J. Ceram. Soc. vol 72, p 725, 1989. Additionally, non-homogeneity will not be eliminated but in fact will be accentuated by such furnace treatment. Most often combustion synthesis by its very nature occurs by propagation of combustion fronts which are spatially marginally unstable. Only in very rare cases such as when TiC is synthesized, is the front completely stable. The instabilities are minor and may not always be apparent to the naked eye, but they do exist. The final part moreover possesses minute bands or other types of non-homogeneities which make electrical and magnetic properties non-uniform. After furnace sintering such non-homogeneities will persist and even amplify into several zones with gross discontinuities.
Outgassing merely holds porosity constant. No actual densification of compact occurs, except to acconmmodate change in molar density from slurry phase to product phase. Thus outgassing only avoids large pores.
When dealing with parts.:which look like wires or thin plates (see U.S. Pat. No. 5,484,568, herein after referred to as "the '568 patent" and U.S. Pat. No. 5,449,886, herein after referred to as "the '886 patent") and which are made from pliable pasty reactants, is impossible to apply pressure or centrifuge without seriously damaging the part in question. In addition, in cases where little or no liquid is created during combustion, the liquid filling technique is impossible to invoke.
Other patents disclosing molybdenum disilicide containing ceramic or metal ceramic composites are: U.S. Pat. Nos. 5,376,421, Dec. 27, 1994; 5,374,342, Dec. 20, 1994; 5,364,513, Nov. 15, 1994; 5,364,442, Nov. 15, 1994; 5,340,448, Aug. 23, 1994; USP 5,340,014, Aug. 23, 1994; 5,316,718, May 31, 1994; 5,310,476, May 10, 1994; 5,279,737, Jan. 18, 1994; 5,217,583, Jun. 8, 1993; 5,188,678, Feb. 23, 1993; 5,127,969, Jul. 7, 1992; and 5,110,688, May 5, 1992. Finally, PCT/US95/04417 filed on Apr. 11, 1995, discloses a reduced pest ceramic, intermetallic or metal ceramic composite including a compound selected from the group consisting of compounds between molybdenum and silicon, tungsten and silicon, and at least 0.5 percent by weight excess added elemental silicon than that required for formation of the compound. Also disclosed in such PCT application is a reduced pest, ceramic, intermetallic or metal ceramic composite including ternary compounds and mixtures thereof selected from the group consisting of (ZAl.sub.x Si.sub.y), where Z is an element and where x is an integer and y is a whole number. All these patents and the PCT application are incorporated by reference herein in their entirety.